FTP.ftp_RETR() should pass FTPCmdError errors back to the client
|Reported by:||exarkun||Owned by:||spiv|
(github, coverage, patch, buildbot, log)
This is like #4909. Any error encountered while reading a file and sending it to a client, to respond to a RETR command, gets logged and a CNX_CLOSED_TXFR_ABORTED is sent back to the client. If the failure is an FTPCmdError though, it can be presented to the client to give them more information about what went wrong.